1. Field of the Invention
The present invention generally relates to a constant-voltage circuit device for a whole category of electronic equipment aboard a computerized personal organizer, a handset, a voice recognition device, a voice memory device, or a computer, etc.
2. Discussion of the Background
Generally, in constant-voltage circuit devices, when an output capacitor has discharged almost all the electricity stored therein at the start-up, electrical current flows to the output capacitor until it is charged with a sufficient amount of electricity. This charge current at the start-up is hereinafter referred to as “inrush current Irush”. How the inrush current Irush occurs and problems caused thereby are described below with reference to FIG. 1.
FIG. 1 illustrates a known constant-voltage circuit device using a series regulator.
Referring to FIG. 1, the known constant-voltage circuit device includes an output transistor M101 that is a PMOS (P-channel Metal Oxide Semiconductor) transistor, a reference voltage circuit 1Z that generates a predetermined or given reference voltage Vref, a differential amplifier circuit 2Z, and resistors R101 and R102 used to detect an output voltage.
The differential amplifier circuit 2Z includes NMOS (N-channel Metal Oxide Semiconductor) transistors M102 and M103, PMOS transistors M104 and M105, and a constant-current source I101 that receives a constant current from a constant-current circuit. Gates of the NMOS transistors M102 and M103 serve as input terminals of the differential amplifier circuit 2Z. The PMOS transistors M104 and M105 together form a current mirror circuit.
The gate of the NMOS transistor M102 serves as an inverting input terminal to which the reference voltage Vref is input, and the gate of the NMOS transistor M103 serves as a non-inverting input terminal. The resistors R101 and R102 are connected in series to an output side of the output transistor M101 and divides an output voltage Vout into a divided voltage Vfb. The divided voltage Vfb is given to the non-inverting input terminal of the differential amplifier circuit 2Z.
The differential amplifier circuit 2Z amplifies differences between the divided voltage Vfb and the reference voltage Vref and outputs the amplified difference to a gate of the output transistor M101. Thus, the output transistor M101 is controlled so that the output voltage Vout output therefrom is kept at a given constant voltage.
Additionally, an output capacitor C101 is externally connected to the output side of the output transistor M101 to smooth the output voltage, and the output transistor M101 is provided with an overcurrent protection circuit 3Z that controls the gate of the output transistor M101 when an output current Iout exceeds a limit current ILMT, thereby controlling the output current Iout.
In this known constant-voltage circuit device, when no or only an extremely small amount of electricity is stored in the output capacitor C101 at the start-up, the output side has an extremely low impedance. Accordingly, a charge current, that is, the inrush current Irush flows until the output capacitor C101 is charged with a sufficient amount of electricity, and then the impedance of the output side becomes high. An upper limit of the inrush current Irush equals the limit current ILMT set by the overcurrent protection circuit 3Z, and a time period during which the inrush current Irush flows depends on the capacity of the output capacitor C101 as well as the limit current ILMT.
FIGS. 2A, 2B, and 2C respectively illustrate waveforms of a power source voltage Vdd; the reference voltage Vref and the output voltage Vout; and the output current Iout in the known constant-voltage circuit device shown in FIG. 1 at the start-up. The output current Iout is the sum of the inrush current Irush and a load current Iload. The waveforms shown in FIGS. 2A, 2B, and 2C are obtained when the power source voltage Vdd is 3.0 V, the output voltage Vout is 1.2 V, the reference voltage Vref is 1.0 V, the output capacitor C101 is 0.5 μF, Rout is 120Ω, and the limit current ILMT is 400 mA.
As shown in FIG. 2B, the reference voltage Vref rises relatively promptly, and accordingly the output voltage Vout rises in a relatively short time. At this time, an inrush current Irush of about 3 Z mA flows because the electric current flows to the output capacitor C101 abruptly.
When the capacitance of the output capacitor C101 is 10 μF, the waveforms of the power source voltage Vdd, the reference voltage Vref, the output voltage Vout, and the output current Iout shown in FIG. 2A through 2C change to those shown in FIG. 3A through 3C.
Although the inrush current Irush may be as large as several amperes if the overcurrent protection circuit 3Z is not provided, the inrush current Irush shown in FIG. 3B depends on the limit current ILMT set by the overcurrent protection circuit 3Z.
When the output voltage Vout rises abruptly as shown in FIG. 3B, the inrush current Irush flows from the power source voltage Vdd to the output capacitor C101 until the output capacitor C101 is sufficiently charged. Although, more precisely, the sum of the inrush current Irush and the load current Iload flows to the output capacitor C101, the load current Iload at the start-up is generally so small as to be negligible compared to the inrush current Irush.
Moreover, if the power source voltage Vdd has a current capacity lower than the inrush current Irush, the power source voltage Vdd will decrease, and there is a possibility that all the circuits connected in parallel to the constant-voltage circuit might fail to start up. Although this inconvenience may be solved by increasing the current capacity of the power source voltage Vdd, the cost of the constant-voltage circuit device will increase accordingly, which is undesirable.
Because the electrical current supplied by the output transistor M101 changes from the inrush current Irush to the load current Iload determined by a load resistor Rout at the moment the output voltage Vout reaches a given intended voltage, the differential amplifier circuit 2Z fails to promptly control the output transistor M101, causing the output voltage Vout to overshoot. As a result, noise is generated in later-stage circuitry, which can invite malfunction of the device.
Although this problem may be solved by increasing the response speed of the differential amplifier circuit 2Z, the electricity consumed by the overall constant-voltage circuit will increase accordingly. In addition, although the overshoot of the output voltage Vout can be reduced by increasing the capacity of the output capacitor C101, which is known when the waveforms shown in FIGS. 2A through 2C are compared with those shown in FIGS. 3A through 3C, increasing the capacity of the output capacitor C101 means that the inrush current Irush flows for a longer time period, and accordingly the power source voltage Vdd will decrease, which is undesirable.
In view of the foregoing, known power source devices include a soft start function so that the output voltage can be gradually increased by gradually increasing the voltage input thereto at the start-up.
However, in such known power source devices, because the reference voltage Vref is switched between multiple voltages using a switch, noise is generated, which can invite malfunction of the device.
Therefore, there is a need to provide a voltage generation circuit that can restrict the inrush current at the start-up with a simple configuration without increasing the electrical consumption and can raise the constant output voltage without overshooting.